What do I use to get the bolts out of the exhaust manifolds on a 87 tristar with a 351 indmar? I never took them out of my old boat but one manifold started cracking. I am going to need to go buy the right size fitting to get this out but dont know the size?

I ran antifreeze through the system though. What size bolt is it? I am going to replace them with the skidm's kit.